The metering of fluid flows is a vital part of many engineering processes control and is therefore directly related to safety and efficiency. Clearly, it is vital for industry to have as accurate and reliable flow meter as possible for many applications. The reality is of course that no meter is ideal for any application and industry is always searching for combinations of cheaper, more reliable, accurate meters that have diagnostic abilities.
In its most basic sense, a DP flow meter combines Bernoulli's theorem (i.e. the conservation of energy of a fluid flow) and the conservation of mass of a fluid flow between two points in a flow, and the difference in pressure between these two points is measured so that a mass or volume flow rate can be expressed as a function of this differential pressure and the fluid density. A DP meter comprises an obstruction to fluid flow and means for measuring the pressure change caused by the obstruction, giving associated flow rate equations for either volume flow rate or mass flow rate wherein these respective flow rate equations are both functions of the fluid density. The obstruction is defined by a “primary element” which can be either a constriction formed in the conduit or a structure inserted into the conduit. The primary element can be for example a Venturi constriction, an orifice plate, a cone shaped element or other form. DP meters are generally held not to refer to laminar flow elements.
Pressure tappings are inserted upstream from the primary element and at or in the vicinity of the primary element and the pressure difference between the two points is monitored. The primary element causes a drop in pressure, and the pressure is measured at the point of the conduit that corresponds to the lowest pressure. For a Venturi meter, this measurement point will correspond to the “throat” of the primary element, that is, the point of the element that has the minimum cross sectional area. (If the precise position of the lowest pressure is not known for a particular geometry of primary element (as for example in the case of Orifice Plate Meters) it is customary to select a stated position where it is known the pressure will be significantly lower than the pressure tapping upstream of the primary element.)
The standard DP meter flow equation is well known, and is described below in Appendix 1. A further type of DP meter flow equation was described in the published PhD thesis, “Wet Gas Metering” by Richard Steven, University of Strathclyde, Glasgow, United Kingdom, April 2001, which is incorporated herein by reference. In this thesis, the concept of a recovery differential pressure was introduced and discussed. The derivation of the associated recovery DP meter flow equation is summarised in the attached Appendix 2.
A “single phase” flow normally means that the fluid in the conduit comprises only one of gas or liquid. However, for some two phase or multi phase flows, if you have a very high pressure and high gas flow rate and the liquid is a light hydrocarbon (or other liquid with suitable low interfacial tension and viscosity) the liquid can be a forced into very fine droplets. That means the wet gas is a mist flow where the liquid has been effectively atomised and the flow is homogenous. Other types of liquids under certain conditions can be forced into very fine droplets. These types of homogenous two-phase or multi-phase flows are also considered to be “single phase” for the purposes of this invention, as the calculations referred to will hold true for such flows in addition to flows which are purely liquid or gas, as will be apparent forthwith.
While the above meters give the basis for many industrial applications, there is always a desire for meters to be more accurate, cheaper, smaller, or a combination of all of these.
There is also a major problem with the diagnosis of a defective meter, in terms of identifying that there is an error in the readings in the first place, and then in determining where the fault is, correcting the readings to take account of the fault, and then in fixing the fault.
Currently, the diagnostics used with flow meters depend on the individual type of meters in question. Some are better than others but industry generally wishes improvements in all meter type diagnostics and practical improvements at reasonable costs is keenly received.
Ultrasonic meters are relatively expensive devices but one of the selling points is that they have more diagnostic abilities than the rival meters. The ultrasonic meter diagnostic procedures wholly consist of analysing and comparing the different information from different ultrasonic wave paths and checking the health of each signal (e.g. attenuation of the signal, the gain required, the speed of sound estimation etc.). From analysis of each paths output ultrasonic meters can give information on the validity of each ultrasonic path readings and decide whether to accept a particular path information or not. If a path is rejected there is mathematical methods used to account for this in the flow computer. The path information also gives through the speed of sound information regarding the chemical make up of the flowing fluid.
However, most other common flow meters in use have significantly less or no practical diagnostic abilities. They are in effect “dumb” devices that offer a flow rate and it is up to the user to judge the validity of the output. This is usually done by comparing the meter readings to other information in the whole system in which they are one component. Many flow meter designs in many different industrial applications operate in error for various reasons for significant periods of time before it is discovered. This can lead to processes being conducted at less than optimum efficiency, safety issues, sales arguments amongst buyers and sellers etc.
One of the most popular meter types is the differential pressure flow meter hilt despite this meter types popularity and wide spread use across all industries there is very little in the way of diagnostics for this meter type. The standard usage is to install the meter with a flow computer with sufficient care that the system should work correctly. Periodic re-calibrations of the secondary devices are carried out to remove any drift on the differential pressure transducers. Periodically a boroscope (i.e. a small camera) may be inserted to check there is no damage to the primary element nor any solid partical build up around the primary element or pressure ports or any objects obstructing the primary element (such as rocks, welders gloves, welding rods, broken valve parts etc.) Periodically the meter will be removed and inspected for damage or wear and recalibrated as there is currently no way to check if it is working correctly other wise. A saturated DP, transmitter is when the actual DP through the meter is larger than the maximum the transmitter can measure so as it reads the maximum it can show and not the real larger DP (unless the flow computer is programmed to check this which is rare) the flow output is in error. Periodically the meter will have the pressure ports “rodded” (i.e. rods forced through the pressure ports to assure that there is no build up of foreign matter clogging the impulse lines (i.e. the tubing between the meter and the DP transmitters). In the case of high temperature applications (especially steam flows) the flowing fluid can not come into direct contact with the DP transmitter diaphragms as the excess temperature would damage the instrumentation so a buffer of liquid columns are used in the impulse lines. However, where as it is recommended practice to use a device called a “condensate pot” (which is a relatively large vessel semi filled of liquid) between each impulse line and the meter to assure no significant condensation and evaporation height change in the impulse line liquid column, in a remarkable percentage of such applications the condensate is dispensed with and through either cost cutting or ignorance of the possible consequences the impulse lines are filled with liquid with no condensate pot. The result is often variations in liquid column height on each impulse line due to evaporation or condensation and a random wondering differential pressure read by the transmitter as the relative liquid column heights move around. There are no accepted reliable and cheap diagnostics for these situations.
In summary, faults in DP meter readings can arise from a variety of causes including damage, wear or partial blockage of the primary element or when a pressure tapping becomes defective for any reason, for example if it is leaking or is blocked. These could be due to waxing, hydrates, a maintenance engineer accidentally leaving the valve on the transmitter manifold closed, lack of a condensate pot where on is required and so on.